Immunization Issues

Thimerosal in Influenza Vaccines

Updated: Octubre 22, 2007

Although thimerosal—a mercury-containing preservative—continues to be used as a preservative throughout the world1, in 1999 policy makers in the United States decided to reduce the exposure of infants less than 6 months of age to all forms of mercury, recommending that thimerosal in vaccines to be given to infants under 6 months of age be reduced to only trace amounts or be eliminated altogether. (See Mercury in Vaccines)

All vaccines recommended for universal immunization of infants less than 6 months of age in the United States are now manufactured without thimerosal as a preservative. Some vaccines may contain trace amounts of thimerosal from the manufacturing process.2

Trivalent inactivated influenza vaccine (TIV) has been recommended for all children 6 months to five years of age, in addition to the long-standing recommendations to give TIV to children and adults with certain medical conditions as well as older adults.3

Until recently, thimerosal-free TIV was not available in the United States (it was.available both with trace amounts of thimerosal and with thimerosal as a preservative). Without thimerosal as a preservative, the vaccine cannot be packaged in multiple dose vials, making delivery of vaccine to large numbers of people—as is often done with TIV—more difficult and making the vaccine more expensive. Packaging vaccine in single dose containers also takes more vaccine and thus may reduce the availability of vaccine for others.

In September, 2007, the FDA approved a new influenza vaccine for persons 18 years of age and older, available in single-dose, thimerosal-free syringes. It is still not known when this new vaccine will be widely available to the public.

Public health authorities have not expressed a preference for which formulation healthcare providers should use because:

  • there continues to be no scientific evidence that thimerosal as a preservative causes harm
  • the amount of mercury contained in TIV with preservative is within the range that is considered safe
  • TIV is not recommended for children less than 6 months of age.

Parents who would prefer that their child receive TIV without preservative can choose to have their child receive the vaccine that contains only trace amounts.

However, the amount of thimerosal in current inactivated influenza vaccines is so low that it should not raise concern. The issues in 1999 regarding thimerosal in childhood vaccines was for infants in the first 6 months of life and reflected uncertainty about the possible cumulative total ethyl mercury burden from all the thimerosal-containing products administered at newborn, 2, 4 and 6 months of age.

Legislation to prohibit use of TIV containing thimerosal in children and others—as has been enacted in some states—could compromise the availability of TIV for those people, particularly during time of vaccine shortage. In addition, the cost differences for the preservative-free vaccine could create disparities between insured and underinsured children receiving the vaccine.

The demonstrated benefits of influenza immunization far outweigh any theoretical risk from thimerosal exposure associated with full implementation of current US influenza vaccine recommendations.3